1. Field of the Invention
The present invention relates to a method of producing medium-grade coated paper for rotogravure printing, the base paper of which contains high-yield pulps. More particularly, it relates to a method of producing medium-grade coated paper for rotogravure printing, which paper, when printed, has fewer gravure dots and reproduction of gravure dots is improved.
2. Description of the Prior Art
Recently the importance of coated paper for printing as a medium for magazines, advertising, catalogs and other publications, is again being recognized. Particularly, the demand for coated paper of grades No. 3, 4 and 5 (hereinafter referred to as "medium-grade coated paper") has been increasing rapidly because of the need of reducing the weight and cost of paper. The trend to lighter weight publication grades is pointed out in various articles, for example, in an article by James P. Hanson, Pulp and Paper magazine, May 1977, pages 74-76. The various grades are identified on page 74. Generally, medium-grade coated paper is widely used in the field of light-weight paper of 45 to 80 g/m.sup.2 as compared with high-grade (grade No. 1 or No. 2) coated paper which does not contain high-yield pulps. Therefore, to compensate for the reduction of opacity resulting from the decrease of the basis weight of the paper and to reduce the cost of paper, the base paper of medium-grade coated paper contains one or more of the high-yield pulps set out below in an amount equal to between 5% and 100% by weight of its pulp composition. The high-yield pulps, and the designation of each, are as follows:
______________________________________ mechanical pulp MP stone-ground pulp SGP pressure stone-ground pulp PSGP refiner-ground pulp RGP thermo-mechanical pulp TMP chemi-mechanical pulp CMP chemi-thermo-mechanical pulp CTMP chemi-ground pulp CGP semi-chemical pulp SCP ______________________________________
In many cases, medium-grade coated paper, because of its purposes and applications, is subjected to rotogravure printing or web offset printing, both being high-speed printing. In rotogravure printing, unlike other printing processes such as offset printing and letter press printing, missing gravure dots and defective reproduction of the dots are liable to be seen because the rotogravure printing is a unique printing process of transferring printing ink directly from an intaglio or a cell on a metal roll to the paper. These are serious disadvantages producing unfavorable printing results.
Missing gravure dots, or speckle, is seen both on high-grade and medium-grade coated papers. It is particularly liable to occur on medium-grade coated paper, because high-yield pulps contained in the base paper thereof contain more shives and bundles than chemical pulps. Even their single fiber pieces tend to be rigid owing to their high lignin content, and sufficient smoothness is not obtained even under the pressure conditions in supercalender finishing and gravure printing. Remedies proposed heretofore include methods of removing shives and bundles from high-yield pulps, methods of decreasing long fiber fractions, improvements on gravure inks and improvements on gravure plate making, but none of them have been very successful.
The cause of defective reproduction of gravure dots is as follows: In rotogravure printing, the ink, highly diluted with organic solvent, is transferred from an intaglio or a cell to coated paper surface and therefore the ink spreads on the coated paper surface at the time of transfer. The defective reproduction of gravure dots results in low quality reproduction of the original material. As it is attributable to the basic characteristics of rotogravure printing, no decisive remedies therefor have been obtained as yet.
The inventors have made a study not only of the base paper but also of a coating composition in order to decrease missing gravure dots and improve the reproduction of gravure dots on medium-grade coated paper in rotogravure printing. As a result, the inventors have successfully obtained medium-grade coated paper, on which missing gravure dots are decreased and the reproduction of gravure dots is improved, by comprising the base paper of high-yield pulps having a certain fiber length distribution and by using a coating composition containing certain natural ground calcium carbonate in a limited proportion and synthetic resin emulsion as main adhesive.